It is well known that photovoltaic cells use specially prepared semiconductor junctions to convert energy from sunlight to electricity.
A problem with solar arrays is the difficulty and expense of making the semiconductor materials. The sun yields a certan amount of energy per square foot of the earth's surface, which for a given latitude and time of day and year is fixed; although it can be diminished to some degree by adverse weather conditions. Consequently, it is necessary to collect the sunlight on an area large enough to yield the desired amount of electricity since the efficiency of even the best semiconductor material in converting sunlight to electricity is limited.
One solution to this problem has been the use of light concentrating devices. Many forms of concentrators have been proposed and some are in use. Some concentrators depend on the use of a lens to focus the sunlight on a photovoltaic cell. Other concentrators use mirrors for the same purpose. By either of these approaches sunlight from a large area can be collected and converted by one or more cells having a much smaller area. The ratios run from 5-1 to as much as 1000-1 in some cases. This approach then is based upon the idea that it is cheaper to cover a substantial portion of the earth's surface with mirrors or lenses than with photovoltaic cells. One undesirable feature of such concentrators is that they require a mechanism to point the apparatus accurately at the sun. This in turn involves the use of moving parts, a sensing system or other form of control. Furthermore, on cloudy days this type of concentrator may be of little use because while there may be considerable light it is diffuse and cannot be readily focused.
It is also well known that a specific photovoltaic semiconductor junction utilizes, for conversion to electricity, only a portion of the spectrum of energy available in sunlight. For example, the conversion of sunlight to electrical energy using well known silicon photovoltaic cells is strongly dependent upon the conversion of light with energy at or above 1.1 electron volts while most of the lower energy light also present is either transmitted through the cell or converted to heat instead of electricity. The heat generated can reduce the efficiency of the silicon cell for the conversion of the higher energy light to electricity.
Some of the sunlight which penetrates the light transmitting member of a conventional nonconcentrating photovoltaic array is lost such as by passing through an interstice between adjacent photovoltaic cells, reflection back out of the member, and the like.
Accordingly, current photovoltaic arrays receive more energy input from sunlight than they retain for conversion into electrical output and it is highly desirable to increase the amount of sunlight an array retains for such conversion.
One known approach for increasing the capacity of photovoltaic arrays to convert sunlight energy to electricity is to employ one or more luminescent agents such as dyes, pigments, metal oxides, and the like in the light transmitting member, which agents, when exposed to sunlight, take in light from one direction and emit lower energy light in numerous directions. An example of such an agent is organic dyes such as the dyes heretofore employed in the scintillation counters, lasers, and the like.
The light falling on the flat surface of a light transmitting member which contains such dyes is absorbed by the dye and re-emitted in an isotropic configuration. In effect, this shifts the wave length of the light to a lower energy and at the same time directs most of the light toward the edge face of the member and, therefore, the edge faces appear quite bright. Thus, the prior art has heretofore required that photovoltaic cells be mounted on the edge faces in order to pick the light up. Since light transmitting members are usually relatively thin sheets, the edge faces of such sheets generally cover a much smaller area than their flat large area sides and the result is a light concentrator.
The particular luminescent agent or agents employed in conjunction with an array of specific photovoltaic cells are chosen, inter alia, for their ability to emit light in an energy level range which suits the conversion characteristics of that cell. This way, a portion of the light that would otherwise be lost for electrical generation, e.g., lost by transmission, reflection, and the like, is transformed by the luminescent agent into multidirectional light that is more readily retained in the light transmitting member by internal reflection, thereby increasing the efficiency of the array.
A photovoltaic array which employs this approach is hereinafter referred to as a luminescent photovoltaic array. Such an array usually employs fluorescent dyes, fluorescence being that species of luminescence wherein the emitted light is usually in the visible spectrum. However, other species of luminescence exist. Phosphoresence (light emission continues after the stimulating light has stopped) is one such species.
It should be understood that this invention covers all species of luminescence, as well as all types of luminescent agents.
Heretofore the prior art has required the photovoltaic cell or cells employed to be mounted on the thin edge face or faces of the light transmitting luminescent member as opposed to its large area sides because internally reflected light tends to concentrate at their edges. However, edge face mounting can be a problem because it is physically difficult to mount photovoltaic cells on the edge of a sheet. Prior art techniques favor the mounting of cells on large flat surfaces. The attendant wires, encapsulation, etc., that go with an array also are not compatible with the use of the edge of a sheet unless the sheet is very thick. A thick sheet, however, is expensive, which in turn obviates much of the benefit of the use of a concentrator.
When using a scintillator to detect a specific particle, the scintillator is excited when the particle in question passes through it and emits light which is guided to a photomultiplier. Since the goal is to pick out a particular particle, the timing as to when excitation occurs and light is emitted is very important. A sharper time pulse can be obtained by recovering the light pulse from an edge of the scintillator. A sharp time pulse cannot be recovered from a large area side of the scintillator. Thus, all such scintillators (collimators) use edge recovery.
In this invention timing of light pulses is not as important as collecting and keeping (concentrating) all the light possible for recovery at the photovoltaic cells. Thus, recovery from a large area side, contrary to timing scintillators, is more desirable.